This invention relates to an electromagnetic beam focusing type cathode ray tube and more particularly to an improvement on a structure for magnetically focusing plural beams in a three in-line gun type color picture tube.
The magnetic beam focusing structure of the three in-line gun color picture tube disclosed in the above-referenced application is a developed version of the magnetic beam focusing system used in a single-electron-gun picture tube described in the Japanese Utility Model Publication No. 26274/75 and controls the three electron beams produced by the three electron guns in such a manner that the beam spots of the three beams on the phosphor screen are spaced a short distance from one another and that each of the beam spots on the screen is prevented from being distorted. Such a magnetic focusing system comprises a pair of members of magnetic material arranged in spaced relation to each other in the direction of travel of the electron beams. Each of the members has a plurality of through holes or apertures for passing the respective electron beams. One member has the periphery of the through holes thereof magnetized to a polarity opposite to that of the periphery of the through holes of the other member, thereby forming a magnetic lens between each corresponding pair of the through holes passing the electron beams.
The means for magnetizing the magnetic material members, i.e. magnetic field generating means, is at least one magnet provided in the neck portion of the tube. Alternatively, the means may be a focusing coil or a cylindrical permanent magnet disposed around the neck portion. Since the shapes of the magnetic field generating means are restricted by the limitations to accomodating space, weight and working technique, as shown in FIG. 5 of the above-referenced application, then it is difficult and also impractical in the fabrication of a three in-line gun assembly to arrange the shape of the magnetic field generating means in a symmetrical relation to all the in-line through-holes. Complicated magnet structure makes the cost high. Also, in the case where a magnetic focusing coil is used around the neck portion of the tube, the coil cannot be arranged in a uniform geometrical relation to the in-line through-holes. Namely, a uniform magnetic flux distribution is not provided over all of the three in-line beams. Accordingly, the focusing magnetic field for the center beam established between the peripheries of the paired through-holes of the magnetic material members by the magnetizing means becomes necessarily different in distribution from the focusing magnetic field for each of the side beams established in a similar manner. In practice, therefore, the focusing magnetic fields deviate from their optimum conditions to some extent. This degrades the focusing quality so that the respective beams cannot be exactly focused on the phosphor screen.